In an ink jet recording method, recording is carried out by jetting fine drops of ink using a variety of mechanisms so as to adhere to a recording paper, and thereby forming ink dots on the recording paper. Therefore, the recording method of ink jet type has advantages in that it is noiseless, can provide full-color prints with ease and enables high-speed printing, compared with the recording method of dot impact type. However, it also has a weak point that printed ink is hard to dry since the ink used for ink jet recording is usually water base ink.
Thus, the paper used in the ink jet recording method is required to have properties of (1) ensuring high-speed drying of ink, (2) providing prints of high optical density, (3) being free from overflowing and feathering of ink, (4) not causing undulation by the absorption of ink, and so on.
It has already been proposed to produce an ink jet recording paper by adopting a cast coating method, thereby answering the above-mentioned requirements to provide printed images of high quality. By using a cast-coated paper as ink jet recording paper, the rippling trouble which has been caused in paper due to expansion just after printing, or the so-called cockling, can be lessened (Japanese Tokkai Sho 62-95285, wherein the term "Tokkai" as used herein means an "unexamined published patent application").
As for the cast coating method, the following three types of processes have so far been known:
(1) a direct process in which the applied coating color in a wet condition is pressed against the hot drum having a mirror-ground surface and undergoes a drying operation, thereby copying the specular plane from the drum surface; PA1 (2) a re-wetting process in which, after the coating color applied in a layer is once dried, a wetting solution is applied to the dried layer to make the layer surface plastic and return it to the wet state, and then the resultant layer is pressed against the hot drum having a mirror-ground surface and undergoes a drying operation, thereby copying the specular plane from the drum surface; and PA1 (3) a coagulation process in which, immediately after a coating color is applied in a layer, the layer is coagulated with a coagulating solution and then pressed against the hot drum having a mirror-ground surface and undergoes a drying operation, thereby copying the specular plane from the drum surface.
That is, in every process of producing a highly glossy cast-coated paper, the copying of a specular plane is effected by pressing the coating color layer against the hot drum having a mirror-ground surface while it is in a wet state and drying it in the pressed condition. Therein, the paper in a wet state is dried as it is stretched on the drum, namely it undergoes tension drying under a bound condition. Therefore, the paper obtained can have excellent dimensional stability.
However, such a cast-coated paper for ink jet recording is attended with a defect of causing curling and rippling troubles during the standing after printing because the paper suffers shrinkage due to difference in the amount of ink applied and recorded area.
In a case where such a cast-coated paper for ink jet recording is used as a substrate to constitute the front part of a pressure-sensitive adhesive sheet, the cast-coated paper part of the pressure-sensitive adhesive sheet shrinks accordingly as the ink printed thereon is dried by leaving the sheet in air, while the release coated paper part of the pressure-sensitive adhesive sheet scarcely causes any dimensional change because the ink used in ink jet recording, in general, cannot permeate into the release coated paper. Therefore, when the irreversible shrinkage takes place to liberate the distortion from the inside of the cast-coated paper, the printed face of the paper generally curls inward with the curling axis coincident with the MD direction (The term "MD direction" as used herein stands for the paper traveling direction in a paper machine, usually called machine direction). As a result, the cast-coated pressure-sensitive adhesive sheet printed thereon by ink jet recording curls so as to direct the cast-coated paper part inward.
Such a curling trouble as described above is serious in the case of using a high-speed ink jet printer which is equipped with a heating roller or infrared heater for rapid drying of the printed ink. In particular, when the system of heating the recorded face from the back side is adopted in the ink jet printer used, the expansibility of a release coated paper forms an important factor, too; as a result, the cast-coated pressure-sensitive adhesive sheet tends to curl with the curling axis coincident with the MD direction, thereby causing traveling troubles. Such a curling trouble, however, cannot be obviated even by a mechanical method using, e.g., a decurler which is known to be effective for the removal of the curling in the CD direction (The term "CD direction" used herein stands for the direction crossing with the machine direction at right angles, usually called cross machine direction). In other words, no effective means to prevent the curling in the MD direction has been found yet (The term "MD direction" stands for the machine direction).